In recent years, the construction of multilayer organic films from polyelectrolytes of alternating charge has received much attention due to the high degree of molecular order induced by interlayer electrostatic forces in these systems. Until now, repeated bilayers have been produced using a variety of methods, including Langmuir-Blodgett deposition, vapor deposition, ionic self-assembly, and drop-casting. For example, ultrathin organic films of alternating charged layers have been constructed by ionic self-assembly, vapor deposition, and Langmuir-Blodgett deposition for use in applications such as biological sensing, optical switching, and waveguiding. Using electrostatic forces to spontaneously induce desired molecular architectures in layer-by-layer organic films has opened up new applications in nonlinear optics for such devices. For example, the ability to modulate a second harmonic signal through the deposition of alternating layers has recently been shown by Casson et al., J. Phys. Chem. B, vol. 104, pp. 11996–12001 (2000). In multilayered films, the effects of the substrate, surface modification, solution pH, deposition temperature, and salt concentration, have been explored extensively.
Deposition of organic material by spin-coating has been done for almost a century. Spin coating, a technique used for casting chemical layers onto a rotating substrate, has been used extensively to prepare thin films for diverse industrial applications such as photolithography, light emission, nuclear track detection, and gas sensing. While the practice of spin coating has existed since the 1920s, mathematical modeling of the spin-coating process began in the late 1950s, when Emslie et al., J. Appl. Phys., vol. 29, pp. 858–862 (1958) described the radial flow of liquids deposited on rotating substrates. Since then, monolayer film formation dynamics has been studied both experimentally and theoretically. Effects of solvent evaporation, liquid viscosity, spin speed, spin time, solute concentration, and solute molecular weight have been examined for a variety of spin-coated systems. The simplicity, time-efficiency, and cost-effectiveness of spin coating make it a practical method for the deposition of polymer thin films. The two main parameters that control the amount of polyelectrolyte adsorbed onto the substrate are solute concentration and spin rate. These have been shown to directly influence the thickness of monolayer films. Dubas et al., Macromolecules, vol. 32, pp. 8153–8160 (1999) have fabricated smoother multilayer polyelectrolyte films by immersing spinning substrates into polyelectrolyte solutions. However, in the present invention, multilayer water-soluble polymer films are spin-assembled by applying, e.g., dropping, a solution onto a spinning substrate.
The present invention involves the ability to control the deposition of water-soluble polymers spin-assembled into multilayer films with monolayer thicknesses on the order of angstroms. The technique of the present invention deposits multilayers onto a substrate in controlled amounts.
A need remains for a procedure of forming multilayer films. After extensive and careful investigation, applicants have now developed an improved process for forming a multilayer thin film heterostructure which shows high control of linear deposition (i.e., thickness, amount and reproducibility).